1. Field of the Invention
The present invention relates to an image pickup apparatus for accumulating a still image of one frame and reading out the accumulated image.
2. Related Background Art
Hitherto, an interline type CCD or the like has been used as an image pickup element for dividing a still image of one frame into two fields and sequentially then reading out.
FIG. 1 is a diagram showing an arrangement of an interline type CCD which can be applied to the present invention. Reference numeral 41 denotes a photodiode; 42 indicates a switch consisting of an MOSFET; 43 a vertical transfer CCD; 44 a horizontal transfer CCD; and 45 an output amplifier.
It is now assumed that photoelectric conversion is executed by the photodiode 41 within a time which has been preset by mechanical shutter means or the like. Then, the photoelectrically converted signal charges are read out from the signal of the first field in a state in which the image pickup element is shut against the light by the shutter means. In this case, the signals of only the odd-number lines (the 1st, 3rd, . . . , (2n-1)th lines) are transferred to the vertical transfer CCD 43 through the switch 42 comprising an MOSFET. Further, the signals are transferred to the horizontal transfer CCD 44 every horizontal line by vertical transfer pulses .phi.V1, .phi.V2, .phi.V3, and .phi.V4. Then, the signals are transferred by horizontal transfer pulses .phi.H1, .phi.H2, .phi.H3, and .phi.H4 and are read out from the output amplifier 45 comprising a floating diffusion amplifier of two source follower type FETs. Next, in the case of reading out the signals of the second field, the signal charges of the even-number lines (the 2nd, 4th, . . . , 2nth lines) are read out in accordance with the same processes as mentioned above. In this manner, the image of one frame is divided into two fields and read out.
Such a conventional reading method has a problem that the dark current noise levels in the first and second fields differ. In other words, when considering the signal charges of the second field, the incident light is shut out by the shutter means for a period of time when the signal of the first field is read out. However, since the signal charges of the second field are held in the photodiode, the dark current increases by an amount corresponding to only such a period of time.
Consequently, there are problems that not only do the S/N ratios of the signals of the first and second fields differ, but also a difference occurs between the black levels, so that flicker occurs and it becomes very hard to see the still image.